Pressure-sensitive adhesives (PSAs) are semi-solid state materials that adhere to a material under lower pressure, are viscoelastic materials, different from adhesives, have basic properties such as initial adhesion, adhesion, and cohesion, and are used in almost all industries such as trademarks of products, advertising, printing, chemistry, medical supplies, home appliances, vehicles, office supplies, and the like.
PSAs may be classified as acrylic, rubber-based, silicon-based and EVA-based PSAs according to monomers used in preparation of PSAs and may be classified as a solvent type, an emulsion type, and a hot-melt type according to the shape thereof.
Most of conventional PSAs used in adhesive tapes, adhesive labels, and the like may be rubber-based PSAs and solvent-type PSAs. However, as demand for environmentally friendly PSAs is increasing, interest in non-solvent type PSAs is increasing and development of technologies therefor is underway. Thus, currently, development and production of non-solvent type PSAs are significantly increasing and will continue to increase. Non-solvent type PSAs are polymerized by emulsion polymerization as a representative example.
Acrylic PSAs require high solid content and low viscosity in terms of economic efficiency and enhancement of processability. The following related arts disclose a high solid content, low-viscosity acrylic PSA.
In particular, Korean Patent Application Publication No. 10-2008-0043926 discloses a water-based PSA with high solid content and low viscosity which includes micrometer scale particles or almost micrometer scale particles using two-step seed emulsion copolymerization and has bimodal or multimodal particle size distribution.
Korean Patent Application Publication No. 10-2009-0008702 discloses that an ionic surfactant and a non-ionic surfactant are used together in order to develop a high solid content acrylic emulsion PSA with low viscosity and the surfactants are separately added in a polymerization process.
In addition, U.S. Pat. No. 6,706,356 discloses a method of preparing an emulsion having bimodal or multimodal particle size distribution by supplying a plurality of pre-emulsions including different amounts of monomers as two distinct feeds to a reactor at different feed rates.
The above-described related arts disclose an acrylic emulsion PSA forming a high solid content latex and including micrometer scale or almost micrometer scale particles having bimodal or multimodal particle size distribution.
However, high solid content latexes with various particle shapes have stability problems. When a latex is low in stability, an excess amount of an agglomerate is produced during polymerization and thus a filter has a problem after polymerization and, when stored, the latex also has stability problems and thus may be hardened. Therefore, an emulsifier needs to be further added in order to enhance stability, but this may be a cause of reduction in adhesion. In addition, to maintain peeling strength of bases such as acrylic PSA-coated paper and film labels and adhesive strength of PSAs to a high level, nanometer scale latex particles having monomodal particle size distribution are needed.